1. Technical Field
This invention is applicable to mobile radio communication. This invention relates, more particularly, to a method and apparatus for deciding in which sector of a mobile communication system a mobile station is in, where a radio zone (referred to as a zone herein) covered by one base station is divided into several sectors in operation.
2. Description of the Related Art
In the prior art mobile communication system, a zone is divided into plural sectors and each sector is covered by an antenna having directivity. Such method is advantageous in that interferences between sectors can be reduced by the directivity of the antennas, and the frequency utilization efficiency is improved as the distance between cells which are using the same frequency channels is shortened. In this type of mobile communication system, it is necessary for a mobile station to find out the serial number zone and of the sector in which the station is in at the time to set up a voice channel before the start of communication.
Even in the prior art mobile communication system where a zone is not divided into sectors, a mobile station sometimes must decide in which zone it is in. In the system where different zones are assigned different frequencies respectively, a mobile station scans control channels assigned to the zones, and detects a zone in which the station is in from the frequency (the serial number) of which control channel the station could receive. Similarly, in a different prior art mobile communications system if a zone is divided into sectors, it is possible for a station to discriminate a sector by its frequency. In other words, sectors are assigned with control channels of different frequencies, and a mobile station decides in which sector it is in by detecting the frequency or channel it can receive; or if a plurality of such channels exists, by measuring receiving levels of these frequencies, comparing them and discriminating the one with the maximum reception level.
However, in such a system of locating the sector in which one mobile station is in from the sector frequency, as sectors are assigned with a control channel of different frequencies, the number of frequencies for control channels is much larger than the number required for the system in which control channels are assigned to different zones. For instance, if the zones are assigned with control channels of different frequencies in the unit of nine zones, and the same frequency is repeatedly assigned to the next unit of nine zones or the 10th zone, then the number of necessary frequencies is nine. But if the sectors are assigned with different frequencies, a zone is divided into six sectors, and the zones are grouped in the unit of seven, 42 frequencies (6.times.7=42) will be required for control channels as the same frequency is reassigned only to the 8th zone. Even if the system has a comparatively simple structure as above, the required number of control channels is as much as five times more.
Moreover, the number of control channels which one mobile station receiver must scan for discriminating the sectors is large. Further, as it generally takes a certain amount of time to monitor the receiving level of a channel, the time required from activation of the power switch of a mobile station to when communication is ready with a set up control channel becomes inconveniently long.
In the above conventional system, moreover, when one mobile station moves across the boundary of sectors, the number of frequencies it must scan in order to detect the next control channel is so large that the station takes much time for detection and control of movement between sectors to conduct switching between the control channels smoothly. As the mobile station moves further during the delay in detection and control, the receiving strength becomes lower than the marginal level at which a mobile station could receive control signals. Occasionally, switching of control channels becomes inoperable. Because one control channel can only cover a narrow area in such a system, the frequency of channel switching increases, and the possibility of not receiving the paging signals during such channel switching increases proportionally. Due to the above reasons, reliability of receiving the paging signals and channel switching control is deteriorated.
In such a system, the same information is often transmitted to all the sectors such as in the case of paging. In such a case, efficiency in utilization of frequency is low as the sectors must be assigned with different frequencies.
In order to overcome the defects of the system where sectors are assigned with different control channels, there has been proposed a practical system where each zone is assigned with control channels of the same frequency. However, the system cannot rely on the aforementioned method to locate a sector where a mobile station exists simply by scanning the frequencies of control channels and detecting the frequency with the maximum receiving level. Instead, a base station measures an upward control channel signal issued from a mobile station and compares with other signals to locate the relevant sector. (The signal generated by a mobile station for a base station is called an upward signal while the one generated from the base station for mobile stations is called a downward signal). In these systems where locating of a sector should be done on the side of the base station, when a mobile station turns on the power source at the start, or when the zone where it is in needs to be located, the station first scans the frequencies of control channels assigned to different zones, measures and compares the receiving levels thereof, discriminates the control channel frequency which has the maximum receiving level, and uses the frequency of the upward control channel corresponding with the discriminated frequency. When a mobile station receives or originates a call, as the mobile station uses an upward control channel to transmit a signal to the base station, the base station receives the upward signal by using an individual receiver allocated to each sector. The base station measures and compares the levels at which each of the receivers receives the signal, detects the sector in which the relevant mobile station is in from the number of the individual receiver with the maximum level. In this system, the burden imposed on the base station becomes great as the individual receivers for the sectors of a zone should measure and compares the levels.
When an upward signal is received to measure its level, if the length of the signals generated from a mobile station is too short, it cannot be measured sufficiently. Due to the changes in level by fading, sectors sometimes cannot be precisely discriminated. In the system where the bit transfer speed of signals is set at a high rate, the signal length generally becomes short to deteriorate precision in sector discrimination. Poor precision in the sector discrimination means a high possibility for connecting a wrong remote base station. In other words, the zones should be defined larger equivalently. This makes the distance which permits reassignment of the same frequency longer, the frequency utilization efficiency lower, and the capacity of subscribers of the whole system incapable of extension.